1. Scrapers
   • Examples
   • Scraper Tractive Forces
   • Excavating Equipment

Scrapers

Scrapers can load, haul & unload material by themselves.

Productivity flexibility is maximum since one piece of equipment being down does not affect the production rate of the other equipment.

Economic haul distances can exceed one mile.

Types of scrapers:

~ crawler tractor towed:

•four-wheeled, two-axle

•pulled by crawler tractor

•short haul, high traction, high drawbar pull, no pusher

~ wheel tractor towed:

•two-wheeled, single-axle, gooseneck

•scraper gooseneck bears weight on tractor drive wheels

•longer haul, higher speed, lower traction, may need pusher

•single engine: engine on tractor only (Fig 6-1, pg. 145)

•twin engine: second engine on scraper driving scraper axle (Fig 6-2, pg, 146)

•multiple units:

-two-bowl tandem:single tractor pulling two scrapers

-"scraper train":units such as in Fig 6-2 together in a train for special, large quantity earth-moving jobs

-push-pull:two tractor scraper units coupled during loading only to eliminate a pusher dozer (one unit loads at a time)

•elevating:

-power and chain driven elevator slats cut and load earth into scraper bowl (Fig 6-3, pg. 146)

- self-loading in most materials (no pusher)

Scraper capacity rating:

~ struck capacity

~ heaped capacity 1:1, 2:1, 3:1 (horiz:vert)

~ ratings in loose cubic yards (lcy):

• bcy = lcy × (swell factor)* (sf range .63-.89)

• swell factors (see Table 4-1, pg 70)

*non-elevating scraper: add 10% of the swell factor to the swell factor to compensate for the forced packing of earth into the bowl.

~ elevating scrapers will only be heaped capacity rated

Scraper operation:

~bowl cutting edge is lowered and front apron is raised to load the scraper bowl

~ apron is closed and bowl raised to haul

~bowl cutting edge height is set, apron raised and earth pushed out by an ejector during unloading

Increasing scraper performance:

~ well laid out worksite & organized work plan

~ well trained & briefed employees

~ well maintained & serviced equipment

~ ripping tight soils before scraping

~ prewetting soil for easier laoding

~ load scrapers down grade

~ use optimum type & quantity of equipment

Cycle time:

~ fixed elements:

• loading

• maneuvering: spot, wait, turn

• accelerate/decelerate

• unloading-dumping (Table 6-1, pg 151)

~ variable elements:

• hauling

• returning

~ cycle time is influenced by haul road condition:

• rolling resistance

• bouncing spills load

• equipment & operator fatigue

• extra equipment downtime

~ loading methods:

• self loading

· elevating scrapers

· crawler tractors

• push loading methods (Fig. 6-7, pg 154)

· pusher back-track loading (longest times)

· pusher chain or shuttle loading (shorter times)

• load-growth curve (Fig. 6-9, pg 157)

· payload is a function of loading time

· point of diminishing return

· locating optimum loading time (Table 6-2, pg 156)

· loading time is also a function of haul distance (Fig. 6-10, pg 158)

~ other considerations:

• informed operators (i.e., work plan, plotting)

• speed of equipment being matched

• equipment standardization

• preventive maintenance & care of equipment

• safety program

Examples

EXAMPLE:wheel-type, single engine tractor scraper combination, non-elevating but push loaded operation under these speed & distance favorable conditions:

Haul Road Section	A	B	C
length (ft)	2,000	1,400	1,100
hauling (mph)	15	8	13
returning (mph)	15	22	13

Variable portion of cycle:

~ haul time = (2,000/15 + 1,400/8 + 1,100/13) × 1/88 = 4.5 min

~ return time = (2,000/15 + 1,400/22 + 1,100/13) × 1/88 = 3.2 min

Fixed portion of cycle: (See Table 6-1, pg 151)

~with 11.4 mph, enter Table 6-1, get 1.8 min for favorable conditions

Total cycle time (T_s):

~ 4.5 + 3.2 + 1.8 = 9.5 min

60-min hour trips:

~ 60/9.5 = 6.3 trips/hr

Scraper production (single scraper):

~ given: capacity = 38 lcy; % swell = 30%

• note (1.1) adds 10% to the swell factor

~ given: 50 min/hr operating efficiency (o.f. = 0.83)

~ 32.2 bcy/trip × 6.3 trips/hr × 0.83 = 168 bcy/hr

Number of scrapers served by a single push-dozer:

Given:

~back-track loading with favorable conditions

~ pusher tractor cycle time, T_p = 1.7 minutes

• T_p = 1.7 min n = Ts/T_p

• Note:figure the costs for both 5 & 6 scrapers to justify the most economical number. If 5 scrapers, dozer waits; if 6 scrapers, scraper waits.

• 5 × 1.7 min. = 8.5 min, means dozer waits 1 min./cycle

• 6 × 1.7 min. = 10.2 min, means scrapers wait 0.7 min./cycle

Scraper production (5 scrapers):

~ 168 bcy/hr × 5 = 840 bcy/hr

Scraper production (6 scrapers)(note: now have t_s = 10.2 min):

~ 32.2 bcy/trip × (60/10.2) trips/hr × 0.83 × 6 = 943 bcy/hr

Production costs:

~ given: scraper w/oper = $45/hr

dozer w/oper = $28/hr

Therefore, use 5 scrapers for this job.

Scraper Tractive Forces

A self-loading scraper is limited in the material it can load by the following traction relationships:

T_max =max. force or tractive effort the engine can deliver to the drivers (rimpull)

T_L =tractive effort available for load

T_RR = tractive effort to overcome rolling resistance

T_GR = tractive effort to overcome grade resistance

W_D = weight on drives

C_t = coefficient of traction

T_A = tractive effort actually applied

T_S = tractive effort at point of drive wheel slippage

Have the limiting relationship for engine power (T_max ) & slippage:

T_max _ T_A _ W_D × C_t = T_S

T_A = T_RR + T_GR + T_L

Excavating Equipment

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